Puffer interrupters have enjoyed ever increasing commercial success. This is due in part to their simple construction and excellent service record. The increased use of puffer interrupters in power class circuit breakers has been at the expense of the more complex two-pressure interrupters. Unfortunately, as opposed to a two-pressure interrupter, a puffer interrupter requires a relatively large prime mover. In particular, the operating force required from the prime mover increases with the capacity of the interrupter. This is because, generally speaking, the interrupter increases in physical size as the choking arc current of the electrical arc increases. Accordingly, the prime mover must be more powerful.
The prime mover mechanism does not operate just during fault situations. In fact, during the life of an interrupter, a majority of time the puffer is opened with little or no load across it's contacts. Thus, for the most part, the prime mover or operating mechanism for a puffer interrupter is over-sized or is provided with excess capacity relative to what is needed to achieve interruption.
A relatively recent advance in design of circuit interrupters has been the so called "self-extinguishing" puffer interrupter. Here the pressure raising function of the electrical arc, due to the thermal energy released by the arc, is used to provide a source of high-pressure gas which is released through the arcing region to extinguish the arc. Those skilled in the art know that high-temperature arc extinguishing gas has a lower density which promotes ionization and has a relatively low insulating capability. In other words, higher temperature fluid has a higher conductivity and a lower capacity for extinguishing an arc. Thus, in a "self-extinguishing" puffer interrupter, large-capacity current interruption is difficult to obtain if the volume of fluid is kept constant. This limitation tends to off-set the benefits gained by being able to use a smaller prime mover. Several patents have been granted for inventions, the purpose of which is to improve the arc extinguishing capability during high current carrying conditions: U.S. Pat. Nos. 4,221,943; 4,225,762; 4,239,949; 4,242,550; 4,243,860; 4,253,002; and EPC No. 00 19 806.
From the foregoing it should be clear that high-pressure can be easily obtained when the arc current is large and that, as the arc current decreases, the pressure necessary to cause interruption becomes more difficult to achieve. This is because when the current is small, the pressure produced within the chamber surrounding the arc is generally insufficient to produce interruption (i.e., the volume is fixed and the thermal energy released is small). Several inventions have been patented whose purpose is to provide adequate arc extinguishing capability in the low current range: U.S. Pat. Nos. 4,259,555; 4,270,034; and 4,327,263. Examination of these patents and those previously cited will show that a satisfactory solution to the problem of providing a self-generating gas flow interrupter which performs satisfactory in both the high current and low current ranges has not yet been achieved. An inexpensive, relatively simple design for a self-generating gas puffer interrupter, which performs satisfactorily in both the high and the low current ranges, will be a welcome addition to the art.